Mixed refrigerant cycle

ABSTRACT

A process for cooling and condensing a first mixture by means of a second multicomponent mixture. The second mixture is cycled through a plurality of heat exchange stages in convective heat transfer relationship to the first mixture. The second mixture is withdrawn from an intermediate heat exchange stage and compressed at low temperatures. The refrigeration supplied to warmer exchange stages in the prior art by the second mixture is supplied according to the new process by a separate refrigeration system or by a subsidiary refrigeration loop within the main second mixture refrigeration system.

Bodnick et al.

1 1 May 20, 1975 [54] MIXED REFRIGERANT CYCLE 3,581,511 6/1971 Peck62/11 3,593,535 7/1971 Gaumer [75] mentors? Bmimck, Rocklfway; 3,702,06311 1972 Etzbach 62/40 Thomas M. Stark, Morr1stown, both of NJ; FOREIGNPATENTS OR APPLICATIONS [73 Assigneez Exxon Research & Engineering895,094 /l962 United Kingdom 62/40 Company, Linden, NJ. PrimaryExaminer-A. Louis Monacell [22] 1972 Assistant Examiner-Hiram H.Bernstein [21 Appl. No.: 304,276 Attorney, Agent. or Firm-Harold N.Wells Related U.S. Application Data [63] Continuation-impart of Ser. No.9.499, Feb. 9, 1970, [57] ABSTRACT abandoned A process for cooling andcondensing a first mixture by means of a second multicomponent mixture.The [52] Cl. 62/9; 62/40; Second mixture is cycled through a pluralityof heat [51] II}!- Cl F251 3/00 exchange Stages in convective heattransfer relation [58] held of Search 62/9 1 ship to the first mixture.The second mixture is with- 62/27 drawn from an intermediate heatexchange stage and compressed at low temperatures. The refrigeration[56] References cued supplied to warmer exchange stages in the prior artby UNITED STATE PA the second mixture is supplied according to the new3,274,787 9/1966 Grenier 62/28 process by a separate refrigerationsystem or by a sub- 3.364,685 1/1968 Perret 62/9 sidiary refrigerationloop within the main second mix- 3 418,819 12/1968 Grunberg..... 1. 62/11 lur refrigeration system, 3,578,073 5/1971 Bosquain 62/40 35815106/1971 Huches 62/40 8 Claims, 4 Drawing Figures m0 m2 104 I06 MIXEDREFRIGERANT CYCLE CROSS REFERENCE TO RELATED APPLICATIONS Thisapplication is a continuation-in-part of application Ser. No. 9499 andnow abandoned.

BACKGROUND OF THE INVENTION In the prior art several methods areemployed for the liquefaction of natural gas feeds. Among these is amulticomponent refrigeration cycle in which natural gas feed at ambienttemperature, normally about 100F., is successively cooled and liquefiedin a plurality of heat exchange stages, resulting in a liquefied naturalgas product at a temperature of about 260F. The cooling medium is amulticomponent refrigerant.

In the prior art method this multicomponent refrigerant having given upits residual cold in a first heat exchange stage, enters a compressor inthe vapor phase, at a pressure of about 1-5 atmospheres and atemperature essentially ambient. By ambient temperature is meant theaveragetemperature of the surrounding environment and thus, as appliedto process streams, it is the temperature which can be closelyapproached by contacting those streams with the air, water, etc. Therefrigerant is pressurized and cooled by heat exchange with water or airto form a two-phase mixture at a temperature slightly above ambient.This two-phase mixture is cycled to a separation drum upstream of thefirst heat exchange stage. The two phases are separated in this drum andboth vapor and liquid phases enter the first heat exchange stage alongwith the natural gas feed. All of the above three streams are cooled bythe recycled multicomponent refrigerant stream. This recycledrefrigerant stream enters the first heat exchange stage as a two-phasemixture countercurrent to the above three streams, generally at F. orbelow. In the course of cooling the three streams, the recycledrefrigeration stream is warmed, exiting as a gas as discussed above at atemperature about ambient. Typical of the prior art method is US. Pat.No. 3,593,535 to Gaumer et al.

The disadvantages of this prior art method lie first in the high powerrequirements of the refrigeration compression step. A seconddisadvantage is the high capital cost of the heat exchange stages. Allthe heat exchange stages in the prior art method are constructed of highcost alloy materials. This is necessary in order to insure that all heatexchanges and heat exchange stages operate without danger of rupture inthe low temperature environment that they are sub ected to in the priorart process.

SUMMARY OF THE INVENTION The method of the instant invention is directedto a process for cooling and condensing a gaseous mixture ant stream isless than the work saved by lowering the compressor section temperature,the process of the instant invention results in considerable powersavings.

The instant invention is also directed to a process in which the capitalcosts are lower than the equivalent prior art process. Thus, in theinstant invention the first heat exchange stage is constructed of carbonsteel rather than high cost alloy materials. This is due to the methodof the instant invention wherein the first stage heat exchanger nolonger contacts a refrigerant stream so cold that carbon steel is notsuitable.

In accordance with the instant invention a first mixture, i.e., thenatural gas feed, is cooled and condensed in a series of heat exchangestages by means of a'second mixture which acts as a refrigerant. Thefirst mixture and the second mixture are both cooled in a first heatexchange stage, both entering at essentially ambient temperature andleaving said stage below ambient temperature. The second mixture is thenseparated into liquid and gaseous phases after leaving the first heatexchange stage. Thereafter, the first mixture and the liquid and gaseousphases of the second mixture are further cooled in a second heatexchange stage by means of a cold recycle of the second mixture. Thegaseous phase of the second mixture which is cooled in the second heatexchanger is again separated into gaseous and liquid phases. The stepsof cooling the first mixture and the liquid and gaseous phases of thesecond mixture by means of a cold recycle of the second mixture and thenseparating the cooled gaseous phase of the second mixture into liquidand gaseous phases are repeated until the first mixture is cooled andcondensed to the desired temperature. The cold recycle of the secondmixture after exiting the second heat exchanger is compressed whilestill below ambient temperature and cooled and thereafter cycled backinto the first heat exchange stage thus completing the cycle. Therefrigeration value lost by recycling the cold second mixture issupplied by one of several alternative methods.

In a preferred embodiment the first and second mixtures are cooled inthe first heat exchange stage by means of a separate refrigeration cycleusing, instead of a multicomponent mixture, an essentially singlecomponent refrigerant.

In another preferred embodiment the cold recycle exiting the second heatexchange stage is compressed in two stages, A portion of the secondmixture is compressed to an intermediate pressure, cooled to nearambient temperature, and then recycled to the first heat exchange stage,thereby providing the cooling for the remainder of the second mixtureand all of the first mixture.

In still another preferred embodiment a portion of the second mixtureafter leaving the first heat exchange stage is recycled and flashed backthrough the first heat exchange stage thus providing a third means ofcooling the first and second mixtures in the first heat exchange stage.

The method of the instant invention requires lower power requirementssince the recycle is compressed after leaving the second heat exchangestage at a much lower temperature rather than at the ambient temperaturetypical in the prior art in which the recycle continued on through thefirst heat exchange stage. Additionally, since the cold recycle is sentto the compressor after leaving the second heat exchange stage, thefirst heat exchange stage is designed with a separate cooling systemwhich is independent of the colder temperatures existing downstream inthe higher heat exchange stages. Hence, the first heat exchange stagemay be constructed of carbon steel since the temperature in the firstheat exchange stage can be designed to never encounter temperaturesbelow F.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understoodby reference to the accompanying drawings of which:

FIG. 1 is a flow diagram of a multicomponent refrigeration cycle with anexternal high level refrigerant cooling the first heat exchange stage;

FIG. 2 is a flow diagram of a multicomponent refrigeration cycle withcooling in the first heat exchange stage provided by expansion of theintermediate pres sure second mixture liquid;

FIG. 3 is an expanded view of a portion of FIG. 2 showing an alternatemethod of heat transfer in the first heat exchange stage;

FIG. 1 as a countercurrent stream) to the streams that are cooledtherein, at a temperature in the range of 30 to 0F. This refrigerationstream exits stage 100 through conduit 76 as a vapor near or slightlybelow ambient temperature. It passes to the remainder of therefrigeration cycle, which is not shown in the drawing, and returnsafter heat has been rejected to the first heat exchange stage 100 as aliquid through conduit 75.

The first mixture flows from the first heat exchange stage 100 into asecond heat exchange stage 102 by means of a conduit 4. It is cooled inexchanger 102 to about 100F. The first mixture leaves exchanger 102through conduit 8.

The second mixture stream which exits the first heat exchange stage l00through conduit 26 as a two-phase mixture is separated into liquid andgaseous phases in FIG. 4 is a flow diagram of a multicomponent refrig- 1eration cycle with cooling in the first heat exchange stage provided byan expansion and recycle of the second mixture liquid exiting the firstheat exchange stage.

DETAILED DESCRIPTION Referring to FIG. 1, in detail, reference numeral 2denotes a conduit supplying a first mixture, which in a preferredembodiment is a natural gas multicomponent feed stock at about ambienttemperature, typically about 100F., into a first heat exchange stage100. If the gas is not already at near ambient temperature, it will beprecooled by a indirect heat exchange against environmental streams,usually air or water, since this is more economical than cooling byrefrigeration. The first mixture is cooled to a temperature of about 0F.and exits through conduit 4. A second mixture which comprises amulticomponent mixture of nitrogen, methane, ethane, propane, butane andheavier hydrocarbons leaves compressor 130 through conduit 22 as agaseous mixture at a pressure of about 350 to 500 pounds per squareinch. The high pressure refrigerant stream thereafter is cooled to nearambient temperature and partially condensed against air or water in anafter-cooler 132. This second mixture then flows through conduit 24 intothe first heat exchange stage 100, cocurrent with the natural gas feedstream. Along with the natural gas multicomponent mixture, this secondmulticomponent mixture is cooled further in stage 100. The two-phasestream exits stage 100 at a temperature of about 0F. through conduit 26,thereafter entering a separation drum 110.

The two above-described streams entering stage 100 through conduit 2 andconduit 24, respectively, are cooled by convective heat transfer. In apreferred embodiment, a plurality of streams are used to cool the firstand second mixtures. Each stream comprises the same single componentrefrigerant but each operates at a different pressure from the others sothat a plurality of boiling temperatures for the same refrigerant isattained. The separate refrigeration cycle removes heat from the ambienttemperature incoming streams and rejects it to the environment. Thisexternal cycle may operate with any single component refrigerant, butthe temperature range suggests that propane would be a preferredrefrigerant. The external refrigerant enters stage 100 either cocurrentor countercurrent (shown in a separation drum 110. The vapor phase ofthe multicomponent second mixture is conveyed from the drum 1 10 to heatexchange stage 102 through conduit 28, entering stage 102 cocurrent withthe natural gas feed. It is therein cooled to a temperature of about F.exiting said stage 102 as a two-phase mixture through conduit 34.Conduit 34 is in communication with a separation drum 112 into which thetwo-phase mixture is discharged. The liquid and gas phases are separatedtherein.

The liquid phase constituent of drum is conveyed therefrom to heatexchange stage 102 through conduit 30. Conduit 30 feeds this liquidstream cocurrently into stage 102 with the natural gas stream and thevapor phase of the refrigerant to stage 102. The liquid is chilledtherein to a temperature of about 100F.

The cooling medium for these three above-described cocurrent streamscomprises in part the liquid phase of the second mixture. This streamexits the second stage 102 through conduit 36 and is flashed in anexpansion valve 120. The resultant flashed, two-phase mixture, at 1 atemperature of about 100 to 130F. and pressure of from I to 5 atm, flowsfrom the expansion valve into a conduit 38. This two-phasemulticomponent stream thereupon enters a conduit 74, where it mixes withthe exiting multicomponent refrigeration stream of the second mixturefrom a downstream heat exchange stage 104 at approximately the sametemperature. The combined stream flows through conduit 74 into the stage102. This cooling stream is heated in the stage 102 thereby vaporizingthe mixture. The recycle stream leaves stage 102 through conduit 20 at atemperature of about 20F. to 10F. and a pressure of 1 to 5 atm. Thestream flows through conduit 20 back to the compressor 130. As will bedescribed hereinafter, the second mixture recycle stream includes secondmixture streams recycled from downstream heat exchange stages. Thus, itis apparent from a simple material balance that the second mixture vaporstream in conduit 20 includes all of the second mixture that enters intothe second heat exchange stage through conduits 28 and 30.

In another preferred embodiment the liquid phase of the second mixtureexiting separation drum 110 is immediately flashed, combined with thecold essentially vapor recycle portion of the second mixture, and passedthrough heat exchange stage 102 countercurrent to the first mixturestream and the vapor phase of the second mixture entering stage 102through conduits 4 and 28, respectively. It should be appreciated thatthis alternate method may be applied to any or all of the heat exchangestages in which the liquid phase of the second mixture is flashed andpassed countercurrently to the first mixture. This alternate method isapplicable not only to FIG. 1 but to the embodiments disclosed in FIGS.2, 3 and 4 hereinafter.

The first mixture in conduit 8 next enters into the third heat exchangestage 104 at a temperature of about lF. A second cocurrent stream entersheat exchange stage 104 through conduit 40. It represents the overheadvapor phase of the second mixture contained in separation drum 112. Athird inlet stream into stage 104 is the liquid phase of the secondmixture contained within drum 112. It enters stage 104 through conduit42. The three streams are cooled to a temperature in the range of 1 60to -2l0F. by a flashed recycle stream of the second multicomponentmixture which flows countercurrently in stage 104, to theabove-described streams. The liquid phase stream exits stage 104 into aconduit 46 which leads the liquid stream into an expansion valve 122wherein the liquid is flashed resulting in a two-phase mixture at atemperature range of about 160 to 230F. and a pressure of latm. Thistwo-phase stream leaves valve 122 through conduit 48 and combines with acooling stream exiting a downstream heat exchange stage 106. Thecombined two-phase multicomponent second mixture stream enters heatexchange stage 104 through a conduit 70 countercurrent to theabove-described warmer streams, wherein the combined stream is heated toa temperature in the range of 100 to 130F. This combined stream exits asa vapor or a two-phase mixture through conduit 72. It is then combined,with the exiting flashed stream from conduit 38, in conduit 74 to formthe inlet refrigerant to exchange stage 102.

The cooled first mixture stream leaves stage 104 by way of a conduit 12.The cooled gaseous phase stream entering through conduit 40 is cooledand substantially condensed exiting stage 104 through conduit 54.

Since the overhead vapor phase is substantially condensed in the stage104, there are only two streams cooled in a fourth heat exchange stage106. They are, respectively, the natural gas multicomponent stream(first mixture) which enters stage 106 through conduit 12 and themulticomponent refrigerant stream (second mixture) which enters thefourth stage 106 through conduit 54. Both streams are cooled to about260F. by means of the exiting chilled liquid phase of the second mixturewhich exits exchange stage 106 through conduit 60. Again, the liquidcontained within conduit 60 is flashed to a lower temperature, in anexpansion valve 126, exiting said valve 126 as a two-phase mixturethrough a conduit 62. The refrigerant in conduit 62 is passedcountercurrent to the above two streams exiting as a vapor or two-phasemulticomponent stream by way of a conduit 64 at a temperature of about-1 60 to 230F. It is thereafter combined with the flashed stream exitingconduit 48 to providethe coolant to heat exchange stage 104, saidcoolant stream entering said stage 104 through conduit 70.

The first mixture stream which entered stage 106 through conduit 12exits said stage 106 through conduit 14 as a liquid at a temperature ofapproximately -260F. This comprises the final liquefied natural gasproduct of the above-described method.

It should be appreciated that in the above disclosure and in FIG. 1,convective heat transfer occurs in heat exchange stages. It should notbe inferred that a heat exchange stage is equivalent to a single heatexchanger. On the contrary, a heat exchange stage should be understoodto include one or more heat exchangers of various kinds which may bedisposed in parallel and/or series configurations. This interpretationshould be given also to the disclosure which follows.

Turning now to FIG. 2 in detail, in another preferred embodiment of thisinvention, a first mixture which in a preferred embodiment is a naturalgas feed enters a first heat exchange stage 200 through a conduit 2 atnear ambient temperature, typically about lOOF. A multicomponent secondmixture which acts as a refrigerant enters into stage 200 through aseries of steps starting at a first compressor 230. The multicomponentrefrigerant leaves the compressor 230 through conduit 252 as a gas at apressure of about 100 to 200 psia. This stream thereupon is cooled in anintercooler 232 wherein the temperature is reduced to approximately100F. by indirect heat exchange against air or water at ambienttemperature thereby condensing the higher boiling point components ofthe multicomponent refrigerant. Thus, a two-phase multicomponent mixtureflows out of intercooler 232 into conduit 254. This multicomponentmixture thereupon enters into a stage knockout drum 234 wherein thephases are separated. The vapor phase is further pressurized in a secondstage compressor 240. The vapor phase enters the compressor 240 from thedrum 234 by way of conduit 256 at an approximate temperature of l0OF.,and pressure of 100 to 200 psia. Upon leaving compressor 240, the vaporstream is pressurized to about 350 to 500 psia. The stream is thencooled in an aftercooler 242 by indirect heat exchange against air orwater at ambient temperature, said stream entering said aftercooler 242by way of conduit 258. Part of the multicomponent mixture may becondensed in said aftercooler 242 resulting in a two-phase mixture at atemperature of about 100F. while maintaining essentially constantpressure. The two-phase mixture exits aftercooler 242 by way of aconduit 260. Conduit 260 is in communication with the first heatexchange stage 200. Thus, the two-phase stream flows into stage 200 in adirection cocurrent with the natural gas feed which enters the firststage 200 through conduit 2.

Since the main refrigerant stream returns to compressor 230 from secondstage 102, the cooling stream required for cooling of the twoabove-mentioned streams is provided by means of the second mixtureliquid phase of the stage knockout drum 234. The liquid phase at atemperature of about 100F. and a pressure of 100 to 200 psia isconducted by means of conduit 270 into an expansion valve 280. Thestream is flashed in said valve 280 resulting in a two-phase mixture ata temperature in the range of 30 to 0F. and a pressure of l to 5 atm. Aconduit 274 leads the two-phase stream from the valve 280 into stage 200in a direction countercurrent to the streams entering through consteadof conducting the liquid phase of the second mixture from knockout drum234 into an expansion valve 280 and then passing the resultant twophasemixture through the first heat exchange stage 200 countercurrent to thewarm streams, the liquid phase may alternately be conveyed cocurre'nt tothe warm streams entering through conduits 2 and 260. This is shown indetail in FIG. 3. The liquid phase in drum 234 enters the first heatexchange stage 200 through conduit 271 at the same temperature andpressure previously disclosed for the stream in conduit 270 in FIG. 2.The liquid stream is further chilled to a temperature of about F.exiting stage 200 through conduit 272. It is then flashed in expansionvalve 280 exiting as a two-phase mixture at a temperature of about 0 to30F. and a pressure of 1 to atm. This resultant two-phase stream isconducted by way of conduit 273 through the heat exchange stage 200countercurrently to the three above-described streams exiting as a gasapproaching ambient temperature (slightly below 100F). The gaseousstream leaves said stage 200 by way of conduit 275 and joins the mainrefrigerant stream of line (not shown).

Returning now to FIG. 2, the exiting second mixture stream from conduit276, or in the aternative, conduit 275, enters a conduit 250 thusmerging with the bypass stream exiting heat exchanger stage 102 by wayof conduit 20. The gaseous second mixture stream exiting through conduit20 has been previously thermodynamically defined in the description ofthe embodiment illustrated in FIG. 1. The combined stream in conduit 250is a gas at a pressure of l to 5 atm. This gaseous stream returns now tothe first stage compressor 230 and is compressed thereby completing thecycle.

The natural gas feed leaves the first heat exchange stage 200 throughconduit 4 at the same conditions as described in FIG. 1. The two-phasesecond mixture stream entering the stage 200 from conduit 260 is cooledtherein to a temperature of about 0F. exiting the heat exchange stage200 through a conduit 30 as a two-phase mixture. The second mixturestream enters a separation drum 210. The vapor and liquid phases areseparated therein and are then passed into heat exchanger stage 102 at atemperature of about 0F.

The process previously described in the FIG. 1 embodiment is applicableto the instant embodiment (FIGS. 2 and 3) insofar as FIG. 1 refers toflow streams downstream of the first heat exchange stage. Thus, thereference numerals of the conduits, heat exchange stages and the likehave not been changed in FIGS. 2 and 3. This is not to say theconditions of all streams with the same number are exactly the same asthe same numbered stream in FIG. 1. It is to say that conditions areapproximately the same so that the use of the same numbers are justifiedin view of the insignificant changes in thermodynamic properties in thesame numbered streams. It should further be appreciated that thoseconduits, separators and the like upstream of the secoond heat exchangestage which are numbered with the same numbers in FIGS. 2 and 3 as inFIG. 1 are subject to the same interpretation.

In a third preferred embodiment, illustrated in FIG. 4, a first mixture,again a gaseous feed stream exactly the same as the feed streampreviously described in the previous embodiments, enters a first heatexchange stage 300 through conduit 2. It is cooled in said first stage300 and exits through conduit 4. It should be understood that thetemperature, pressure and the other properties of the firstmulticomponent product stream are the same as previously disclosed inthe embodiment shown in FIGS. 1 and 2.

A second cocurrent stream enters the first heat exchange stage 300through a conduit 342. This stream is the second mixture which enters asa two-phase mixture at approximately F. and 350-500 psia. The two-phasemulticomponent second mixture is further cooled in stage 300 and exitssaid stage 300 still as a two-phase mixture, at a temperature of about0F. The exiting second mixture stream flows through a conduit 344 into aseparation drum 310. The liquid phase contained within said drum 310exits the drum 310 through a conduit 346 which branches into twoadditional conduits. One conduit 348 leads a portion of the liquid intoan expansion valve 380. The liquid stream is therein flashed exiting asa two-phase mixture at a temperature of about 0 to 30F. and anapproximate pressure of 100-200 psia. This second mixture, two-phasestream thereafter is recycled back into the first stage 300countercurrent to the streams entering through conduits 2 and 342. Itcools these streams, thereby absorbing enough heat to exit stage 300 asa gas approaching ambient temperature (slightly below 100F). Thisgaseous stream flows through a conduit 376 from stage 300 to adownstream stage of a refrigerant compressor 330. It should beunderstood that this gaseous stream enters a stage downstream of theinlet of compressor 330 since it is at a higher pressure than the streamentering the inlet of compressor 330. Thus, it requires a lessercompression step to bring it up to the required high pressure. It shouldbe further undertsood that alternately the two-phase gaseous stream ofthe second mixture may alternately enter the inlet of a secondcompressor as will be described hereinafter.

At the same time that the stream in conduit 376 enters a downstreamstage of compressor 330, the cold gaseous recycle stream of the secondmixture in conduit 20 is recycled back to the upstream end of thecompressor 330. It should be understood that the stream in conduit 20has similar thermodynamic properties, as were previously described, tothe stream exiting through conduit 20 in FIG. 1. The stream exitingconduit 20 into the upstream end of compressor 330 is of course at amuch lower pressure than that gaseous stream entering'a downstream stageof the compressor 330 through conduit 376. The combined second mixturegaseous streams exit said compressor 330 as a pressurized gas at apressure of 350 to 500 psia through conduit 340. The gaseousmulticomponent second mixture stream thereupon enters an aftercooler 332wherein it is cooled by indirect heat exchange against air or water andis partially condensed. The two-phase stream leaves said aftercooler 332at a' temperature of about 100F. This two-phase stream flows throughconduit 342 back into the first heat exchange stage 300 therebycompleting the cycle.

In an alternate embodiment the second mixture stream in conduit 20enters a first compressor and is compressed to a pressure equal to thatin the second mixture gaseous stream in conduit 376. Thence, the streamexiting the first compressor is combined with the stream in conduit 376and together they enter the second compressor. Other embodimentsemploying more than two compressors may also be employed. The

alternate embodiment described herein is given by way of illustrationand is not inclusive.

The rest of the process steps are similar to those previously describedin the previous embodiments. Therefore, the numbers assigned to thevarious streams, apparatus and the like in FIG. 4 are the same as thoseused in FIGS. 1, 2 and 3. The temperatures, pressures, and phasesascribed to similarly numbered streams, apparatus and the like areapplicable to'this embodiment, subject to the comments made previouslyas to their approximate applicability. The one exception is the liquidphase second mixture stream entering the second heat exchange stage 102.Unlike the embodiment disclosed in FIG. 1 only a fraction of the liquidphase of the second mixture in separation 310 flows intoheat exchangestage 102. The fraction which is approximately 60 to 90 percent of theliquid phase content of separator 310 flows into the second heatexchange stage 102 from the separator 310 by way of a conduit 350. Theremaining to 40% of the liquid contents of separator 310 is flashed andrecycled into the first heat exchange stage 300 by way of conduit 348.

it should be understood that the above preferred embodiments may bemodified without departing from the scope and spirit of the invention.Thus, other temperatures, pressures and phase conditions may be employedto achieve optimum operating conditions depending upon the particularcircumstances under which the natural gas is to be liquefied.Furthermore, the number of heat exchange stages employed in thepreferred embodiments is illustrative and not limiting. Hence, more orless than four heat exchange stages may be used without departing fromthe scope of the invention.

We claim:

1. A method of cooling and liquefying natural gas in a warm stage by afirst refrigeration system employing a first refrigerant and thereafterfurther cooling andliquefying said natural gas in a cold second stage bya second refrigeration system employing a multicomponent secondrefrigerant, each of said first and second refrigeration systemsdischarging heatreceived from said cooling and liquefying of natural gasin said associated first and second stages by compressing saidrefrigerants and condensing them by indirect heat exchange againstenvironmental cooling streams comprising the steps of: a. precoolingsaid natural gas to essentially ambient temperature by indirect heatexchange against at least one of the group of environmental coolingstreams consisting of air and water and thereafter;

b. cooling below ambient temperature in saidwarm first stage by indirectheat exchange said natural gas and independent therefrom saidmulticomponent second refrigerant by said first refrigerant which exitssaid first stage after being warmed therein and after being compressedis cooled by one of the group of environmentalstreams consist-. ing ofair and water, thereby rejecting heat received from the natural gas andthe second refrigerant in said first stage to the environment and isexpanded and returns to said warm first stage as refrigerant;

c. further cooling and liquefying said natural gas in said cold secondstage by indirect heat exchange with said multicomponent secondrefrigerant which thereafter exits said cold second stage afterabsorbing heat therein at a temperature below that of said natural gasentering said cold stage and enters the second refrigeration systemcompressor suction at substantially its exit temperature and pressureand thereafter is compressed and cooled to near ambient temperature;thereby rejecting heat absorbed in said second stage by indirect heatexchange against at least one of the group of environmental streamsconsisting of air and water and thereafter is cooled below ambienttemperature in said warm first stage of (b) and thereafter separatedinto liquid and vapor phases and is returned to said cold second stagewherein said liquid phase is cooled and thereafter reduced in pressure,and returned through said second stage as a portion of said secondrefrigerant and said vapor phase is cooled. 2. The method of claim 1wherein said first refrigerant substantially comprises propane.

3. A method of cooling and liquefying natural gas in a warm first stageby a first multicomponent refrigerant and thereafter further cooling andliquefying said natural gas in a cold second stage by a secondmulticomponent refrigerant, each of said first and second refrigerantsdischarging heat received from said cooling and liquefying of naturalgas in said first and second stages by indirect heat exchange againstenvironmental cooling streams comprising the steps of:

a. precooling said natural gas to essentially ambient temperature byindirect heat exchange against at least one of the group ofenvironmental cooling streams consisting of air and water andthereafter;

b. cooling below ambient temperature in said warm first stage byindirect countercurrent heat exchange said natural gas and independenttherefrom said second multicomponent refrigerant against said firstmutlicomponent refrigerant which exits said first stage after beingwarmed therein and while at substantially its exit temperature joinssaid second refrigerant leaving said second stage and is compressedtogether with said second refrigerant and thereafter condensed andcooled to near ambient temperature by indirect heat exchange against atleast one of the group of enviromental streams consisting of air andwater and thereafter separated as a liquid from the vaporized secondrefrigerant and thereafter expanded to lower pressure and returned tosaid warm first stage as refrigerant;

c. further cooling and liquefying said natural gas and said secondrefrigerant in said cold second stage by indirect countercurrent heatexchange against said second refrigerant which after absorbing heattherein exits from said second stage at a temperature below that of saidnatural gas entering said cold stage and thereafter without significantintervening temperature change joins said first refrigerant and iscompressed jointly with said frist refrigerant to a pressure at whichsaid first refrigerant is fully condensed and cooled to near ambienttemperature by indirect heat exchange against at least one of the groupof environmental streams consisting of air and water, said first andsecond refrigerants thereafter being separated and said secondrefrigerant being further compressed and cooled to near ambienttemperature by indirect heat exchange against at least one of the groupof environmental streams consisting of air and water and thereafter iscooled below ambient temperature in said wann first stage of (b) andthen separated into liquid and vapor streams and is returned to saidcold second stage.

4. The method of claim 3 wherein said first and second multicomponentrefrigerants comprise mixtures of nitrogen, methane, ethane, propane,butane and heavier hydrocarbons, with the compositions of saidrefrigerants being selected to provide suitable temperatures for coolingsaid first and second stages respectively.

5. A method of cooling and liquefying natural gas in a warm first stageby a first multicomponent refrigerant and thereafter further cooling andliquefying said natural gas in a cold second stage by a secondmulticomponent refrigerant, the heat removed by said refrigerants insaid first and second stages being discharged by cornpressing saidrefrigerants and condensing them by indirect heat exchange againstenvironmental cooling streams comprising the steps of:

a. precooling said natural gas to essentially ambient temperature byindirect heat exchange against at least one of the group ofenvironmental cooling streams consisting of air and water andthereafter;

b. cooling below ambient temperature in said warm first stage saidnatural gas and the combined first and second refrigerants by indirectcountercurrent heat exchange against said first refrigerant which exitssaid first stage after being warmed therein;

c. separating the combined first and second refrigerants into a vaporand a liquid stream and thereafter separating a portion of said liquidstream and expanding and returning said portion to said first stage assaid first refrigerant;

d. further cooling and liquefying said natural gas and the secondrefrigerant consisting of the vapor stream of (c) and the remainder ofthe liquid stream of (c) in said cold second stage by indirectcountercurrent heat exchange against said second refrigerant whichtherafter exits said second stage after absorbing heat therein at atemperature below that of said natural gas entering said cold stage andwithout significant intervening temperature change is compressed andcombined with the warmed first refrigerant from said first stage andthereafter said combined refrigerants are further compressed and cooledto near ambient temperature by indirect heat exchange against at leastone of the group of environmental streams consisting of air and water,said combined refrigerants thereafter passing to said warm first stage.

6. A method of cooling and liquefying natural gas in a warm first stageby a first refrigeration system employing a first refrigerant andtherafter further cooling and liquefying said natural gas in more thanone cold second stage by a second refrigeration system employing amulti-component second refrigerant comprising the steps of:

a. precooling said natural gas to essentially ambient temperature byindirect heat exchange against at least one of the group ofenvironmental cooling streams consisting of air and water;

b. compressing and cooling said second refrigerant to essentiallyambient temperature by indirect heat exchange against at least one ofthe group of environmental streams consisting of air and water;

c. further cooling below ambient temperature the precooled natural gasof (a) and the cooled second refrigerant of (b) by indirectcountercurrent heat exchange in said first stage against said firstrefrigerant, whereby said first refrigerant absorbs heat from saidnatural gas and said second refrigerant and is vaporized and thereafteris compressed and condensed by indirect heat exchange againstenvironmental streams to reject the said absorbed heat to theenvironment, and thereafter is expanded and returned to said first stageto absorb heat to complete a refrigeration cycle;

d. separating the second refrigerant of (c) after exiting said firststage into a liquid stream and a vapor stream;

e. further cooling said natural gas and said separated liquid and vaporstreams of (d) in said second stage by indirect countercurrent heatexchange against said second refrigerant, said second refrigerantabsorbing heat from said natural gas and said liquid and vapor streamsand being vaporized leaves said second stage below ambient temperatureand thereafter is compressed at substantially said exit temperature instep (b) thereby completing the second refrigeration cycle;

f. further cooling said natural gas and the vapor stream of (d) in athird stage, said vapor stream being flashed into liquid and vaporportions before cooling in said third stage, said third stage beingcooled by said second refrigerant depleted of said liqiud stream of (d)which is flashed after passing through said second stage and joins saiddepleted second refrigerant before entering said second stage as saidsecond refrigerant.

7. The method of claim 6 wherein said first refrigerant substantiallycomprises propane.

8. A method of cooling and liquefying natural gas in warm first stage bya first multicomponent refrigerant and therafter further cooling andliquefying said natural gas in a cold second stage by a secondmulticomponent refrigerant, each of said first and second refrigerantsdischarging heat received from said cooling and liquefying of naturalgas in said first and second stages by indirect heat exchange againstenvironmental cooling streams comprising the steps of:

a. precooling said natural gas to essentially ambient temperature byindirect heat exchange against at least one of the group ofenvironmental cooling streams consisting of air and water andthereafter;

b. cooling below ambient temperature in said warm first stage byindirect countercurrent heat exchange said natural gas and independenttherefrom said second multicomponent refrigerant against said firstmulticomponent refrigerant which exits said first stage after beingwarmed therein and while at substantially its exit temperature joinssaid second refrigerant leaving said second stage and is compressedtogether with said second refrigerant and thereafter condensed andcooled to near ambient temperature by indirect heat exchange against atleast one of the group of environmental streams consisting of air andwater and thereafter separated as a liquid from the vaporized secondrefrigerant and returned to said warm first stage and cooled therein andthereafter expanded and returned to said first stage as refrigerant;

. further cooling and liquefying said natural gas and said secondrefrigerant in said cold second stage by indirect countercurrent heatexchange against said second refrigerant which after absorbing heattherein exits from said cold second stage at a temfrigerants thereafterbeing separated and said second refrigerant being further compressed andcooled to near ambient temperature by indirect heat exchange against atleast one of the group of environmental streams consisting of air andwater and thereafter is cooled below ambient temperature in said warmfirststage of (b) andthen separated into liquid and vapor streams and isreturned to said cold second stage.

1. A method of cooling and liquefying natural gas in a warm stage by afirst refrigeration system employing a first refrigerant and thereafterfurther cooling and liquefying said natural gas in a cold second stageby a second refrigeration system employing a multicomponent secondrefrigerant, each of said first and second refrigeration systemsdischarging heat received from said cooling and liquefying of naturalgas in said associated first and second stages by compressing saidrefrigerants and condensing them by indirect heat exchange againstenvironmental cooling streams comprising the steps of: a. precoolingsaid natUral gas to essentially ambient temperature by indirect heatexchange against at least one of the group of environmental coolingstreams consisting of air and water and thereafter; b. cooling belowambient temperature in said warm first stage by indirect heat exchangesaid natural gas and independent therefrom said multicomponent secondrefrigerant by said first refrigerant which exits said first stage afterbeing warmed therein and after being compressed is cooled by one of thegroup of environmental streams consisting of air and water, therebyrejecting heat received from the natural gas and the second refrigerantin said first stage to the environment and is expanded and returns tosaid warm first stage as refrigerant; c. further cooling and liquefyingsaid natural gas in said cold second stage by indirect heat exchangewith said multicomponent second refrigerant which thereafter exits saidcold second stage after absorbing heat therein at a temperature belowthat of said natural gas entering said cold stage and enters the secondrefrigeration system compressor suction at substantially its exittemperature and pressure and thereafter is compressed and cooled to nearambient temperature; thereby rejecting heat absorbed in said secondstage by indirect heat exchange against at least one of the group ofenvironmental streams consisting of air and water and thereafter iscooled below ambient temperature in said warm first stage of (b) andthereafter separated into liquid and vapor phases and is returned tosaid cold second stage wherein said liquid phase is cooled andthereafter reduced in pressure, and returned through said second stageas a portion of said second refrigerant and said vapor phase is cooled.2. The method of claim 1 wherein said first refrigerant substantiallycomprises propane.
 3. A method of cooling and liquefying natural gas ina warm first stage by a first multicomponent refrigerant and thereafterfurther cooling and liquefying said natural gas in a cold second stageby a second multicomponent refrigerant, each of said first and secondrefrigerants discharging heat received from said cooling and liquefyingof natural gas in said first and second stages by indirect heat exchangeagainst environmental cooling streams comprising the steps of: a.precooling said natural gas to essentially ambient temperature byindirect heat exchange against at least one of the group ofenvironmental cooling streams consisting of air and water andthereafter; b. cooling below ambient temperature in said warm firststage by indirect countercurrent heat exchange said natural gas andindependent therefrom said second multicomponent refrigerant againstsaid first mutlicomponent refrigerant which exits said first stage afterbeing warmed therein and while at substantially its exit temperaturejoins said second refrigerant leaving said second stage and iscompressed together with said second refrigerant and thereaftercondensed and cooled to near ambient temperature by indirect heatexchange against at least one of the group of enviromental streamsconsisting of air and water and thereafter separated as a liquid fromthe vaporized second refrigerant and thereafter expanded to lowerpressure and returned to said warm first stage as refrigerant; c.further cooling and liquefying said natural gas and said secondrefrigerant in said cold second stage by indirect countercurrent heatexchange against said second refrigerant which after absorbing heattherein exits from said second stage at a temperature below that of saidnatural gas entering said cold stage and thereafter without significantintervening temperature change joins said first refrigerant and iscompressed jointly with said frist refrigerant to a pressure at whichsaid first refrigerant is fully condensed and cooled to near ambienttemperature by indirect heat exchange against at least one of the groupof environmental streams consisting of air and water, said first andsecond refrigeranTs thereafter being separated and said secondrefrigerant being further compressed and cooled to near ambienttemperature by indirect heat exchange against at least one of the groupof environmental streams consisting of air and water and thereafter iscooled below ambient temperature in said warm first stage of (b) andthen separated into liquid and vapor streams and is returned to saidcold second stage.
 4. The method of claim 3 wherein said first andsecond multicomponent refrigerants comprise mixtures of nitrogen,methane, ethane, propane, butane and heavier hydrocarbons, with thecompositions of said refrigerants being selected to provide suitabletemperatures for cooling said first and second stages respectively.
 5. Amethod of cooling and liquefying natural gas in a warm first stage by afirst multicomponent refrigerant and thereafter further cooling andliquefying said natural gas in a cold second stage by a secondmulticomponent refrigerant, the heat removed by said refrigerants insaid first and second stages being discharged by compressing saidrefrigerants and condensing them by indirect heat exchange againstenvironmental cooling streams comprising the steps of: a. precoolingsaid natural gas to essentially ambient temperature by indirect heatexchange against at least one of the group of environmental coolingstreams consisting of air and water and thereafter; b. cooling belowambient temperature in said warm first stage said natural gas and thecombined first and second refrigerants by indirect countercurrent heatexchange against said first refrigerant which exits said first stageafter being warmed therein; c. separating the combined first and secondrefrigerants into a vapor and a liquid stream and thereafter separatinga portion of said liquid stream and expanding and returning said portionto said first stage as said first refrigerant; d. further cooling andliquefying said natural gas and the second refrigerant consisting of thevapor stream of (c) and the remainder of the liquid stream of (c) insaid cold second stage by indirect countercurrent heat exchange againstsaid second refrigerant which therafter exits said second stage afterabsorbing heat therein at a temperature below that of said natural gasentering said cold stage and without significant intervening temperaturechange is compressed and combined with the warmed first refrigerant fromsaid first stage and thereafter said combined refrigerants are furthercompressed and cooled to near ambient temperature by indirect heatexchange against at least one of the group of environmental streamsconsisting of air and water, said combined refrigerants thereafterpassing to said warm first stage.
 6. A method of cooling and liquefyingnatural gas in a warm first stage by a first refrigeration systememploying a first refrigerant and therafter further cooling andliquefying said natural gas in more than one cold second stage by asecond refrigeration system employing a multi-component secondrefrigerant comprising the steps of: a. precooling said natural gas toessentially ambient temperature by indirect heat exchange against atleast one of the group of environmental cooling streams consisting ofair and water; b. compressing and cooling said second refrigerant toessentially ambient temperature by indirect heat exchange against atleast one of the group of environmental streams consisting of air andwater; c. further cooling below ambient temperature the precoolednatural gas of (a) and the cooled second refrigerant of (b) by indirectcountercurrent heat exchange in said first stage against said firstrefrigerant, whereby said first refrigerant absorbs heat from saidnatural gas and said second refrigerant and is vaporized and thereafteris compressed and condensed by indirect heat exchange againstenvironmental streams to reject the said absorbed heat to theenvironment, and thereafter is expanded and returned to said first stageto absorb heat to complete A refrigeration cycle; d. separating thesecond refrigerant of (c) after exiting said first stage into a liquidstream and a vapor stream; e. further cooling said natural gas and saidseparated liquid and vapor streams of (d) in said second stage byindirect countercurrent heat exchange against said second refrigerant,said second refrigerant absorbing heat from said natural gas and saidliquid and vapor streams and being vaporized leaves said second stagebelow ambient temperature and thereafter is compressed at substantiallysaid exit temperature in step (b) thereby completing the secondrefrigeration cycle; f. further cooling said natural gas and the vaporstream of (d) in a third stage, said vapor stream being flashed intoliquid and vapor portions before cooling in said third stage, said thirdstage being cooled by said second refrigerant depleted of said liqiudstream of (d) which is flashed after passing through said second stageand joins said depleted second refrigerant before entering said secondstage as said second refrigerant.
 7. The method of claim 6 wherein saidfirst refrigerant substantially comprises propane.
 8. A method ofcooling and liquefying natural gas in warm first stage by a firstmulticomponent refrigerant and therafter further cooling and liquefyingsaid natural gas in a cold second stage by a second multicomponentrefrigerant, each of said first and second refrigerants discharging heatreceived from said cooling and liquefying of natural gas in said firstand second stages by indirect heat exchange against environmentalcooling streams comprising the steps of: a. precooling said natural gasto essentially ambient temperature by indirect heat exchange against atleast one of the group of environmental cooling streams consisting ofair and water and thereafter; b. cooling below ambient temperature insaid warm first stage by indirect countercurrent heat exchange saidnatural gas and independent therefrom said second multicomponentrefrigerant against said first multicomponent refrigerant which exitssaid first stage after being warmed therein and while at substantiallyits exit temperature joins said second refrigerant leaving said secondstage and is compressed together with said second refrigerant andthereafter condensed and cooled to near ambient temperature by indirectheat exchange against at least one of the group of environmental streamsconsisting of air and water and thereafter separated as a liquid fromthe vaporized second refrigerant and returned to said warm first stageand cooled therein and thereafter expanded and returned to said firststage as refrigerant; c. further cooling and liquefying said natural gasand said second refrigerant in said cold second stage by indirectcountercurrent heat exchange against said second refrigerant which afterabsorbing heat therein exits from said cold second stage at atemperature below that of said natural gas entering said cold stage andthereafter without significant intervening temperature change joins saidfirst refrigerant and is compressed jointly with said first refrigerantto a pressure at which said first refrigerant is fully condensed andcooled to near ambient temperature by indirect heat exchange against atleast one of the group of environmental streams consisting of air andwater, said first and second refrigerants thereafter being separated andsaid second refrigerant being further compressed and cooled to nearambient temperature by indirect heat exchange against at least one ofthe group of environmental streams consisting of air and water andthereafter is cooled below ambient temperature in said warm first stageof (b) and then separated into liquid and vapor streams and is returnedto said cold second stage.